Electrodialysis is a generally known electromembrane process for transport of ions through membranes as a result of an electrical driving force. In an electrodialytic process using nonselective membranes that are permeable to ions, electrolytes can be separated from nonelectrolytes. If the membranes are more permeable to anions than to cations or vice versa, e.g., ion-exchange membranes, the concentrations of ionic species in solution can be decreased or increased by electrodialysis. Thus practical depletion or concentration of electrolyte solutions is achieved.
In an electrodialyzer a number of cation-selective and anion-selective membranes are arranged between a pair of electrodes so that the electrodes and one or more membranes form a multiplicity of parallel solution channels. With multichannel electrodialysis, irreversibilities represented by decomposition potentials at the electrodes can be distributed over many channels and thus minimized. Problems of handling products formed at the electrodes can be minimized as well.
An electrodialysis stack in an electrodialyzer comprises an alternating array of cation and/or anion permeation membranes which together with end caps and seal supports limit compartments or channels. End caps and seal supports are non-conductive and liquid impermeable and are joined to form an outer boundary of a stack. End compartments or channels are defined by an end cap and a membrane. Disposed within one end channel is a suitable anode and disposed within the opposite end channel is a suitable cathode. Anode and cathode are connected to positive and negative terminals of a suitable electrical power source respectively for a required supply of direct current. The anode-containing channel has flowing therein an anolyte and the cathode-containing channel is for the flow of a catholyte.
An industrial electrodialysis stack can be of any well known type of membrane assembly such as a plate and frame type assembly containing a plurality of planar membranes in parallel spaced relations with about one millimeter space between each membrane. The electrodialysis stack can have a number of repeating separation units which typically vary in configuration from two to five channels per separation unit depending upon the character and nature of the ions being transported for separation, the transporting solvents and electrolytes. The order of cation permeation and anion permeation membranes in each repeating unit will vary with the separation or separations to be effected.
After the recovery of cobalt or cobalt and manganese starting by water extraction of a residue of the process for manufacture of isophthalic acid or for example, from U.S. Pat. No. 2,964,599 or British Patent Specification No. 1,413,829, terephthalic acid is known, while some manufacturers of said phthalic acids use the aqueous extract solution as a source of the cobalt or cobalt and manganese metal oxidation catalyst even though such solution contains small amounts of the oxygenated aromatic compounds and even smaller amounts of compounds having an adverse affect on the quality, especially color, of the phthalic acid product. Other manufacturers of such phthalic acids elect to precipitate metal catalyst carbonate from solution and dissolve the metal carbonate in acetic acid as a means for eliminating the return of oxygenated aromatic Co and by-products to oxidation.
Isophthalic acid or terephthalic acid are manufactured by the air oxidation at a temperature above 150.degree. C. of liquid m-xylene or p-xylene in the presence of an acetic acid solution containing ions of cobalt or cobalt and manganese as oxidation metal catalysts together with a promoter therefor which can be either bromide ions, or acetaldehyde or methyl ethyl ketone. The fluid oxidation effluent, which is a suspension of isophthalic acid (IPA) or terephthalic acid (TPA) as crystalline product in an acetic acid solution containing in addition to catalyst metals inter+ alia+ dissolved IPA or TPA and, oxygen-containing aromatic compounds including benzoic acid one or more of the isomers of toluic acid and carboxybenzaldehyde. The fluid effluent is cooled to a commercially feasible temperature of from 100.degree. C. down to about 50.degree. C. to precipitate additional IPA or TPA and then subjected to solid-liquid separation means to recover a crystalline IPA or TPA product, and acetic acid mother liquor still containing catalyst metal ions, some IPA or TPA and the other oxygen containing aromatic compounds. The acetic acid mother liquor is distilled to remove its water (by-product of the oxidation) and most of the acetic acid as a mixture as feed for fractionation to recover acetic acid of 3 to 5 weight percent water and reject the by-product water. The distillation bottoms product contains about 30% to 40% solids which comprise the catalyst metals, IPA or TPA and the other oxygen-containing aromatic compounds and 60% to 70% by weight solvent which is 90% acetic acid and 10% water. The distillation bottoms is one residue which can be treated according to the process of this invention. But such distillation bottoms can be further heated to evaporate substantially the 70% solvent to a liquid residue fluid at a temperature of 115.degree. to 125.degree. C. Such 115.degree. C. to 120.degree. C. fluid residue can also be treated according to the process of this invention.
The present invention replaces said carbonate precipitation and acetic acid redissolving by electrodialysis and substitutes the consumption of a small amount of electrical energy for the rather large cost of the precipitating sodium carbonates and redissolving acetic acid.
We are not aware of any prior publication which discloses the electrodialysis separation of catalyst metals from the oxygen-containing aromatic compounds or publications from which one skilled in the art of separating such metals from solution also containing the oxygen-containing aromatic compounds could perceive the electrodialysis separation comprising the main feature of the present invention.
Trimellitic anhydride (TMA) is produced from pseudocumene (1,2,4-trimethylbenzene) by its oxidation with molecular oxygen to trimellitic acid (TMLA) in an acetic acid solution containing as components of catalyst the following: ions of bromine in combination with either ions of cobalt, manganese and zirconium, or with ions of cobalt, manganese and cerium, or with ions of cobalt, manganese, cerium and zirconium. Such oxidation processes are described in U.S. Pat. Nos. 3,491,144, 3,532,746 and 3,683,016 all of which are incorporated by reference.
Impure TMLA is recovered from the oxidation effluent, which is typically an acetic acid solution comprising components catalyst, TMLA, mono- and dimethyl benzoic acid, unsubstituted benzoic acid, ortho-, iso- and terephthalic acids, methylphthalic acids, and carboxybenzaldehydes, by crystallization from such solution followed by solid-liquid separation. Impure crystalline TMLA is washed with acetic acid and melted to form impure TMA which is distilled to a high purity product. The acetic acid mother liquor is distilled to remove acetic acid and water as a distillate which is then fractionated to obtain an acetic acid product containing 3 to 5 weight percent water. Residues from distillation of impure TMA and residues from distillation of acetic acid mother liquor contain catalyst components and a substantial amount of TMA as well as materials boiling lower and higher than TMA.
Another technique for recovery of TMA is distillation to remove water and acetic acid from acetic acid solution effluent from the oxidation of pseudocumene. Such distillation also converts TMLA to TMA by dehydration. Thereafter, the resulting mixture, which is impure TMA, is distilled to remove TMA. A modification of the foregoing recovery technique involves adding the acetic acid solution oxidation effluent to liquid impure TMA which is maintained just above its melting point temperature. The acetic acid and water in this case are flashed off as a vapor product which can be used as feed to the fractional distillation recovery of acetic acid from water. Impure TMA liquid is drawn off and distilled to recover high purity TMA.
Both of the foregoing final distillations of impure TMA leave a catalyst metal-containing residue having also oxygenated aromatic compounds boiling higher than the boiling point of TMA. Such a mixture of metal-organics and higher boiling oxygenated compounds must contain a substantial amount of TMA to have the residue liquid at reasonable temperatures, such as 180.degree. C. to 235.degree. C.
Such TMA process residues contain various components of which some are water-soluble and others are water insoluble in from 0.25 up to 6 weight parts, usually 0.6 up to 3.0 weight parts, of water per 1.0 weight part of residue at temperatures of from 25.degree. C. up to 100.degree. C. Extraction of TMA process residues with such amounts of water at said temperatures will dissolve: more than 90 percent of the catalyst metals which are present as organo-salts, substantially all of the inorganic and organic bromides, substantially all of the TMA as its hydrated TMLA triacid analog, and some of the acidic oxygen-containing co-products, e.g., benzoic acid and ortho-toluic acid. The insoluble oxygen containing co-products include iso- and terephthalic acids, toluic acids aldelhydo-benzoic and phthalic acids and carboxy- and aldelhyo-esters which are also oxygen-containing derivatives of pseudocumene.
Many techniques have been proposed for the recovery of catalyst metals from such aqueous extract solutions. For example, sodium carbonate per se or the mixture thereof with sodium bi-carbonate are added to the solution to precipitate at pH of 7 to 8 the carbonates of at least cobalt and manganese. The precipitate is recovered and dissolved in acetic acid to regain those catalyst metals as their acetates for reuse in the oxidation of pseudocumene. This recovers the catalyst metals but not the dissolved TMLA. British Patent Specification No. 1,413,829 describes such a catalyst recovery.
U.S. Pat. No. 4,284,523 which is incorporated by reference, describes a method of treating the TMA process residue with water. The residue is extracted with from 0.35 up to 1.5 weight parts of water per weight part of residue at a temperature of from 70.degree. C. up to 100.degree. C. Then, with or without first separating water-in-solubles, the mixture or first solution is either diluted with additional water without change of the 70.degree. C. to 100.degree. C. temperature and the diluted mixture is cooled to a temperature of 20.degree. C. to 35.degree. C. or the first solution is cooled to a temperature of 20.degree. C. to 35.degree. C. and diluted with additional water without change of that 20.degree. to 35.degree. C. temperature. Such dilutions precipitate dissolved solids. Finally, either of the 20.degree. C. to 35.degree. C. mixtures is subjected to solid-liquid separation techniques to recover some aqueous solution of catalyst metals separate from insolubles and precipitates. In either of said dilutions, the amount of water used is the amount required to percipitate 15 percent to 20 percent of the dissolved high boiling oxygen-containing aromatic compounds. Said technique has for its purpose the limiting of high boiling oxygen-containing aromatic compound contamination of reusable catalyst metals.
The final 20.degree. C. to 35.degree. C. aqueous solution filtrate can be treated with carbonates to first selectively precipitate iron, if present as a contaminating corrosion metal, and then at higher pH precipitate cobalt annd manganese. The 20.degree. C. to 35.degree. C. separated aqueous solution can alternatively be treated with a cation exchanger followed by elution of the exchanger with a strong inorganic acid to remove catalyst metals from the cation exchanger and regenerated as disclosed by U.S. Pat. No. 4,298,759. Also the separated 20.degree. to 35.degree. C. temperature aqueous solution can be contacted with one side of a cation permeable membrane the other side of which is contacted with a hydrohalidic acid whereby the catalyst metal ions migrate through the membrane into said acid which carries the metals as halide salts as disclosed in U.S. Pat. No. 4,311,521.
The foregoing catalyst metal reclaiming process of U.S. Pat. No. 4,311,521 uses the cation permeable membrane in a dialysis separation step, but is not effective with acids other than hydrohalidic acids. The separation process of U.S. Pat. No. 4,311,521 functions on the basis of a transfer of metal cations through the membrane from an aqueous feed solution to the hydrohalidic solution and the transfers therefrom of hydrogen cations through the membrane to the aqueous solution. Such transfers are apparently not dependent on the use of a conductive solution and are not effective when acetic acid is substituted for the hydrohalidic acid solution.
In the manufacture of trimellitic anhydride, the intramolecular anhydride of trimellitic acid or 4-carboxyphthalic anhydride, by the procedure before described, the residue product can contain from 50 to 80 weight percent trimellitic anhydride and can amount to from 16% to 26% of the anhydride product recovered. Such residues can also contain from 0.3 up to 2.5 weight percent cobalt (calculated as the metal) and manganese in an amount of from 0.24 to 0.6 times the cobalt. The cerium, when present, will be about equal to manganese. The zirconium, when present, will be 0.01 to 0.1 times the cobalt content. It is important to recover cobalt from the residue because the cobalt metal has had a market value of forty dollars per pound within the last ten years. Environmental and economic considerations require recovery of catalyst metals.
Trimetallitic anhydride process residues suitable for treatment by th method of the present invention typically contain from 50 up to 85, preferably 65 to 85, weight percent trimellitic acid and intramolecular anhydride as flux for materials boiling higher than the trimellitic anhydrides including the organo-metal compounds containing the extractable catalyst metals. The process is useful for compositions shown in Table I (below):
TABLE I ______________________________________ TRIMELLITIC ANHYDRIDE PROCESS RESIDUE Component Concentration, wt. % ______________________________________ Acetic Acid 0 to 5.0 Benzoic Acid 0 to 5.0 Toluic Acids 0 to 5.0 Phthalic Acids 0.25 to 15. Other Lower Boiling Compounds.sup.1 0.5 to 5. Trimellitic Acid and Anhydride 50 to 85 Other Higher Boiling Compounds.sup.2 0.25 to 5 Cobalt.sup.3 0.3 to 2.5 Manganese.sup.3 0.07 to 1.5 Cerium.sup.3 0 to 1.5 Iron.sup.3 0.01 to 0.06 Zirconium.sup.3 0.01 to 0.25 Sodium.sup.3 0 to 1.2 Bromine.sup.3 0.2 to 2.5 ______________________________________ .sup.1 "Other Lower Boiling Compounds" are those unnamed boiling lower than trimellitic anhydride. .sup.2 "Other Higher Boiling Compounds" are those unnamed boiling higher than trimellitic anhydride. .sup.3 Analysis for sodium by atomic absorption, all of the other element by xray diffraction.